Historically, dental restorations have been created through a lost wax casting technique. The dentist prepares a tooth, grinding it into the required shape to properly accept a restoration (such as a crown, inlay, or bridge). An impression of the prepared tooth and the teeth surrounding it is made. Another impression is made of the opposing teeth that will contact the restoration when it is seated in the patient's mouth. Dental stone is poured into the impressions, forming a set of models. The region of the prepared tooth is manipulated so that it can be removed and reinserted into position relative to the other teeth. A technician creates a wax pattern of the restoration to be cast.
The design of the chewing surface of the wax pattern, and ultimately the casting made from it, are critical for proper function and patient comfort. The chewing surface must properly contact the surface of the opposing tooth when the upper and lower teeth are together, but there must be no interference between the upper and lower surfaces when the jaw moves from side to side during functions like chewing. If interferences occur, teeth become very sensitive and, in some cases, a root canal treatment may be required; the jaw functions improperly, leading to temporalmandibular joint disorders; or one tooth may move, because of resultant forces during function, to a position that is physiologically unstable.
The surface shapes of any dental restoration are unique to the tooth being restored; the surface of the restoration is not simply a reflection of the surface of the occluding tooth. The surface must incorporate morphologic characteristics of the tooth to be restored which are, for the most part, uniform for a given tooth for all people regardless of sex, race, age and other factors. There are generally two to five such tooth forms for each tooth in the mouth. For example, a mandibular first molar has three buccal and two lingual cusps in a relatively fixed relationship to each other for all human first molars. The surface must also incorporate the unique characteristics that permit it to function properly in the patient's mouth. This requires that the positions of cusps and grooves or valleys on the tooth may have to be raised or lowered to provide the appropriate contacts without interference.
In the current state of the art, a pattern (usually wax or some check bite material) is made of the interdigitation between the upper and lower teeth in their static, closed position. Generally little information is gathered directly from the patient about the envelope of motion of lower teeth past the upper teeth during function. Instead, the pattern is inferred from the anatomy of adjacent and opposing teeth and the dentist makes necessary adjustments by grinding the surface of the restoration when it is inserted. This method is time consuming and has questionable accuracy.
In some cases, information about the motion of the jaw is obtained directly from the patient. Two techniques are generally used; (1) a face bow transfer and (2) a functionally generated path. To obtain a face bow transfer, the dentist uses a special device to locate the axis of rotation of the lower jaw relative to the upper jaw. Using this information, the casts are mounted on an articulator that preserves these relationships. The motion of the teeth through their entire range of motion is inferred from the articulator and casts and is used to design the wax pattern for the restoration. This technique is time consuming and may or may not improve the precision of the design of the chewing surface of the restoration.
The functionally generated path technique permits a clinician to obtain information defining a functionally balanced occlusion, unique to each patient, without the need for a dental articulator. This technique is, however, time-consuming, complicated and difficult to master.
The advent of CAD/CAM systems for dental restorations has made automation of the design of dental restorations possible. CAD/CAM dental restoration systems are, for example, described in U.S. Pat. No. 4,742,464 issued May 3, 1988 to Duret, et al, U.S. Pat. Nos. 4,766,704 and 4,837,732 issued Aug. 30, 1988 and Jun. 6, 1989, respectively, to Brandestini et al and U.S. Pat. No. 5,027,281 issued Jun. 25, 1991 to Rekow et al. The system described in the Brandestini patents does not account for the occlusal surface of the tooth with the automation. Instead, the dentist must shape the occlusal surface of the restoration at the time that it is placed in the patient's mouth. With the system described in the Duret, et al patent, the dentist must design the chewing surface of the tooth through a series of interactive manipulations.
By contrast, the system disclosed in the Rekow, et al patent utilizes a suitable probe to take measurements in the patient's mouth of the dental prep and of at least the surrounding teeth. This information is then utilized to modify a standard tooth form for the tooth being restored, dimensions for which are stored in a computer memory in a form suitable for operating a CAD/CAM machine. Finally, this patent teaches a CAD/CAM machine which may be utilized for converting the modified tooth form data into the desired dental restoration.
While the Rekow, et al system permits a restoration to be automatically generated, it is not capable of providing an exact match for the chewing or occlusal surface. This system also does not fully account for functional requirements during chewing and other mouth movements, such requirements including proper angling of the lingual cusp of posterior mandibular teeth. Thus, the dentist or other clinician is forced to compensate for inaccuracies and inconsistencies when the restoration is placed in the patient's mouth by using a drill or other tool to eliminate interferences in mating at the occlusal surfaces and any functional interferences. Depending on the degree of inaccuracy in the originally generated restoration, this process can be time consuming and is unpleasant for the patient.
Further, the stored tooth form must be scaled to the proper dimensions for the patient's mouth. In the Rekow patent, this is done by measuring the gap and performing height and width measurements on adjacent (proximal) teeth, and then using the ratio of these dimensions to the corresponding dimensions of the stored tooth form to produce a scaling factor to be applied to the dimensions of the scaled tooth form. Unfortunately, such gross scaling may not always provide satisfactory results and improved scaling techniques are, therefore, required.
Further, the tooth form needs to be angularly oriented relative to the adjacent teeth in at least three dimensions. While the prior art has oriented the tooth about the depth axis so that the groove in the restoration is aligned with the grooves of the one or more adjacent teeth, the prior art has not specifically addressed the curve of Spee which is the angle of the tooth from the front of the mouth toward the rear of the mouth for posterior teeth, nor has such art addressed the curve of Wilson, the angle of the teeth in a direction away from the cheek and toward the tongue. These angles must be taken into consideration in the modeling of the tooth form in order for a properly fitting tooth to be machined.
A need, therefore, exists for an improved technique for modeling dental restorations from measurements of a patient's mouth and related information to produce CAD/CAM coordinates for fabricating a restoration or other dental prosthesis. Such technique should provide consistent and accurate matching on the occlusal surface so that little, if any, work is required on the restoration after it is placed in the patient's mouth and, in particular, should take into account functional factors on the occlusal surface. Such a modeling technique should also provide more accurate scaling of a stored tooth form model to the dimensions of the patient's mouth and should take into account all of the angular degrees of freedom of the tooth and the unique demands of jaw motion in the modeling thereof.